multi-axis aln-on-silicon vibration energy harvester with integrated ...
MULTI-AXIS ALN-ON-SILICON VIBRATION ENERGY HARVESTER WITH INTEGRATED FREQUENCY-UPCONVERTING TRANSDUCERS J. L. Fu1, Y. Nakano2, L. D. Sorenson1, and F. Ayazi1 1 Georgia Institute of Technology, Atlanta, Georgia, USA 2 Tohoku University, Sendai, Japan
ABSTRACT
This paper presents fully-integrated multi-axis piezoelectric-on-silicon kinetic energy harvesters (KEHs) that demonstrate enhanced power output via mechanical frequency upconversion. Mechanical energy is converted to electrical energy by out-of-plane and in-plane devices that are micromachined on the same substrate. The out-of- plane device demonstrates nearly 100u frequency upconversion of 134 Hz input vibrations, while the in- plane harvester demonstrates more than 3000u frequency upconversion of 2 Hz input vibrations. The batch- fabrication process is compatible with AlN-on-Si devices such as RF resonators and sensors. The total volume of an individual harvester is 5 mm3 (in-plane) and 1 mm3 (out- of-plane), implying that stacked arrays of such devices can easily increase power density.
INTRODUCTION Readily-available low-frequency mechanical energy, such as human walking at 1-2 Hz [1] and other common vibration sources up to 150 Hz [2], can be transformed into useful power for portable and wireless microsystems. However, it is challenging to implement miniaturized energy harvesters at such low frequencies. Piezoelectric cantilever harvesters generate sufficient power to operate wireless temperature sensors [3];; however, maximum power is achieved at the device resonance frequency in 100s of Hz or greater, meaning that lower-frequency vibrations cannot be converted with the same efficiency. Although environmental vibrations occur across multiple degrees of freedom (multi-DOF), most energy harvesters operate as a single-DOF mass-spring system [4]. To completely capture mechanical energy produced by such systems, devices must be duplicated and oriented along different axes, which further increases system size and complicates integration. As the resonance frequency of electromechanical transducers increases, higher power levels can be captured for constant input vibration amplitudes [4]. However, the amount of useful ambient energy decreases with frequency. For this reason, mechanical frequency upconversion has been explored to combine low- frequency ambient energy scavenging with resonant MEMS devices. Recently reported harvesters utilize an inertial mass to collect environmental energy and actuate high-frequency transducers to increase the power output [5, 6]. However, the overall volume, including magnets, is in hundreds of mm3 or larger. To address these challenges, we introduce a multi- axis kinetic energy harvesting (KEH) platform composed of micromachined AlN-on-Si devices (Fig. 1). In-plane and out-of-plane vibrations from 2 Hz to 134 Hz are successfully harvested and upconverted without post- process assembly or large external transducers.
978-1-4673-0325-5/12/$31.00 ©2012 IEEE
Figure 1: Backside view of a silicon die with in-plane and out-of-plane kinetic energy harvesters fabricated side-by- side. PRINCIPLE OF OPERATION KEHs should provide power regardless of device orientation with respect to the vibration source. A multi- axis device can eliminate the need to manually align the harvester for maximum power. To achieve multi-axis energy harvesting, both out-of-plane and in-plane harvesters are realized on the same die. Out-of-Plane Energy Harvester The out-of-plane harvester comprises a seismic mass tethered to the substrate with integrated AlN-on-Si beam transducers. The mass captures low-frequency ambient vibrations in an out-of-plane translational mode (Fig. 2, top). Additionally, motion of the mass couples energy into the transducer high-frequency fundamental clamped- clamped beam mode (Fig. 2, bottom). To sense a high- strain region of the beam deflection, where the most charge is generated by the piezoelectric effect, the transducer electrodes and AlN cover the area near the substrate anchor point.
Figure 2: Out-of-plane harvester mode shapes (fmass = 149 Hz and fbeam = 13.7 kHz) and proposed method of operation.
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MEMS 2012, Paris, FRANCE, 29 January - 2 February 2012
In-Plane Energy Harvester The in-plane harvester consists of a seismic mass with free-standing cantilever micro-SLFNV ȝ-picks) located in the center of the mass (Fig. 3a). The mass exhibits translational motion in response to low-frequency vibrations (Fig. 3b). When external acceleration generates sufficient force, HDFK ȝ-pick can ³VQDS´ DQ $O1-on-Si spring transducer that subsequently vibrates at higher frequencies (Fig. 3c). The transducer top electrode connects regions of identical strain polarity to collect charge (Fig. 3d).
FABRICATION PROCESS FLOW Devices are fabricated on DȝP62,VXEVWUDWH (Figs. 4, 5, 6) using the AlN-on-Si process [7] with two modifications. First, the backside oxide mask is selectively etched to control seismic mass thickness, which enables any number of devices on the same wafer to be customized for a particular operating frequency. Secondly, HF release forms z-shock stops that ensure in- plane harvester ȝ-pick alignment.
Figure 4: Cross-section view of energy harvester on SOI substrate with Mo/AlN/Mo.
(a)
(b)
(c)
Figure 5: SEM images of out-of-plane harvester. AlN and top Mo are patterned to sense the fundamental beam mode, exposing bottom Mo in the beam center.
+ ++ ++++ _ _ _ _ _ _ _ _ _ _ _+ _+_+ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ +++ _ _ __ +++++ + + ++++ + + +_+__ _ _ _ _ _ _ _ _ _ _ _ _ __ +_ _+ + _ + _ + + _ _+ _+ _+ _+ _+ _+ +_ _+ +_ +_ +_ _+ + +_+_ _ _ _ _ +_ + + _ + + +++ + + + + ++++ _ _ _ _ +_ _ _ _ +_+_+ +
+ _ _ +
Relative strain Relative strain max (d) max Figure 3: Conceptual diagram of in-plane harvester: (a) rest position, (b) low-frequency seismic mass vibration, (c) high-frequency spring transducer vibration, (d) spring transducer strain patterns with charge formation for in- plane modes at 2.23 kHz (upper) and 6.24 kHz (lower). min
min
Figure 6: (upper left) SEM image of in-plane harvester ȝ- picks and spring transducer;; (lower left) z-shock stop removed to show handle Si;; (right) backside optical view.
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EXPERIMENTAL RESULTS Fabricated devices are mounted on a stage that provides sinusoidal out-of-plane or in-plane acceleration ain with frequency fin. The harvester output current is fed through a load resistance RL to a transimpedance amplifier (TIA). The TIA output voltage, which is proportional to the converted current, is measured by an oscilloscope. The out-of-plane harvester, with four beam transducer outputs connected in parallel, is first characterized over various accelerations at an input frequency close to the seismic mass resonance. A sinusoidal output signal fout = fin = 126 Hz is generated by the harvester, while increasing ain produces large transient current spikes occurring with frequency fspike in addition to the sinusoidal signal. When ain is increased to 0.7g, fspike also becomes equal to fin (Fig. 7).
Figure 7: Out-of-plane beam transducer output at various accelerations.
The output current impulses are found to have a main spectral component at fring 12 kHz (Fig. 8), which corresponds to the fundamental beam mode. A peak power of 3.23 nW is measured in response to a 134-Hz, 0.6g acceleration input.
Figure 8: Out-of-plane beam transducer output with periodic upconversion spikes;; (inset) close-up view of signal ring-down with 12kHz spectral component. A single spring transducer of the in-plane harvester is connected to the TIA, and similar acceleration characterization is performed at lower frequencies (Fig. 9). Based on preliminary measurements, a minimum acceleration of 0.45g is required to induce periodic actuations of the spring transducer with an fspike equal to fin. Missed actuations may result from imperfect ȝ-pick alignment due to slight tilting of the sample. For ain = 0.45g and fin = 2 Hz, complete actuations of one in-plane spring transducer occur with fspike = fin. A single upconversion spike produces a peak power of 2.27 nW (Fig. 10). Spectral analysis confirms signal components at fring = 2 kHz and 7 kHz contained in the upconversion spikes, which are in close agreement with simulated spring modes. While testing limitations prevented simultaneous measurement of both spring transducers, the total output current should nearly double as a result of the transducer symmetry [6]. The cantilever transducers on the seismic mass, which are also covered with piezoelectric material, can be measured in conjunction with the spring transducer output. Power calculations were based on measured current and RL. While RL = 560 : was used for in-plane acceleration characterization (Fig. 9), measurements with RL values up to 560 k: (Fig. 10) confirm that the output power can be maximized by selecting RL to approach the source impedance of the harvester.
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Figure 10: In-plane spring transducer output with upconversion spikes (RL = 560 k:);; (inset) close-up view of 7 kHz signal ring-down.
ACKNOWLEDGMENTS This project was sponsored in part by the Semiconductor Research Corporation through the Interconnect Focus Center, the National Nanotechnology Infrastructure Network (NNIN, USA), and the National Institute for Materials Science (NIMS, Japan). The authors also thank the Georgia Tech Nanoelectronic Research Center cleanroom staff for fabrication support.
REFERENCES
Figure 9: In-plane spring transducer output at various accelerations (RL = 560 :). CONCLUSION This work introduced micromachined multi-axis AlN-on-Si KEHs as a solution to scavenge low-frequency mechanical energy from the environment. Both out-of- plane and in-plane devices utilized integrated frequency upconversion transducers that increased the power output beyond the already-harvested energy at the tested input frequencies. The fabrication process accommodates for multiple degrees-of-freedom by incorporating out-of-plane and in- plane harvesters on the same substrate. Additionally, the fabrication process allows for multiple operating frequencies (and increased power output) by controlling the oxide mask thickness that determines the seismic mass thickness.
[1] D. E. Newland, "Pedestrian excitation of bridges²recent results," in Tenth Int. Congr. Sound Vib., 2003, pp. 1±15. [2] S. Roundy, "On the Effectiveness of Vibration-based Energy Harvesting," Journal of Intelligent Material Systems and Structures, vol. 16, p. 809, 2005. [3] R. Elfrink, et al., "First autonomous wireless sensor node powered by a vacuum-packaged piezoelectric MEMS energy harvester," in IEDM 2009, 2009, pp. 1-4. [4] C. B. Williams, et al., "Development of an electromagnetic micro-generator," Circuits, Devices and Systems, IEEE Proceedings, vol. 148, pp. 337-342, 2001. [5] T. Galchev, et al., "Non-resonant bi-stable frequency- increased power scavenger from low-frequency ambient vibration," in Transducers 2009, 2009, pp. 632-635. [6] I. Sari, et al., "An Electromagnetic Micro Power Generator for Low-Frequency Environmental Vibrations Based on the Frequency Upconversion Technique," J. Microelectromech. Sys., vol. 19, pp. 14-27, 2010. [7] W. Pan, et al., "Thin-film piezoelectric-on-substrate resonators with Q enhancement and TCF reduction," in MEMS 2010, 2010, pp. 727-730.
CONTACT *J. L. Fu, tel: +1-404-385-6693;; [email protected].
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ABSTRACT
This paper presents fully-integrated multi-axis piezoelectric-on-silicon kinetic energy harvesters (KEHs) that demonstrate enhanced power output via mechanical frequency upconversion. Mechanical energy is converted to electrical energy by out-of-plane and in-plane devices that are micromachined on the same substrate. The out-of- plane device demonstrates nearly 100u frequency upconversion of 134 Hz input vibrations, while the in- plane harvester demonstrates more than 3000u frequency upconversion of 2 Hz input vibrations. The batch- fabrication process is compatible with AlN-on-Si devices such as RF resonators and sensors. The total volume of an individual harvester is 5 mm3 (in-plane) and 1 mm3 (out- of-plane), implying that stacked arrays of such devices can easily increase power density.
INTRODUCTION Readily-available low-frequency mechanical energy, such as human walking at 1-2 Hz [1] and other common vibration sources up to 150 Hz [2], can be transformed into useful power for portable and wireless microsystems. However, it is challenging to implement miniaturized energy harvesters at such low frequencies. Piezoelectric cantilever harvesters generate sufficient power to operate wireless temperature sensors [3];; however, maximum power is achieved at the device resonance frequency in 100s of Hz or greater, meaning that lower-frequency vibrations cannot be converted with the same efficiency. Although environmental vibrations occur across multiple degrees of freedom (multi-DOF), most energy harvesters operate as a single-DOF mass-spring system [4]. To completely capture mechanical energy produced by such systems, devices must be duplicated and oriented along different axes, which further increases system size and complicates integration. As the resonance frequency of electromechanical transducers increases, higher power levels can be captured for constant input vibration amplitudes [4]. However, the amount of useful ambient energy decreases with frequency. For this reason, mechanical frequency upconversion has been explored to combine low- frequency ambient energy scavenging with resonant MEMS devices. Recently reported harvesters utilize an inertial mass to collect environmental energy and actuate high-frequency transducers to increase the power output [5, 6]. However, the overall volume, including magnets, is in hundreds of mm3 or larger. To address these challenges, we introduce a multi- axis kinetic energy harvesting (KEH) platform composed of micromachined AlN-on-Si devices (Fig. 1). In-plane and out-of-plane vibrations from 2 Hz to 134 Hz are successfully harvested and upconverted without post- process assembly or large external transducers.
978-1-4673-0325-5/12/$31.00 ©2012 IEEE
Figure 1: Backside view of a silicon die with in-plane and out-of-plane kinetic energy harvesters fabricated side-by- side. PRINCIPLE OF OPERATION KEHs should provide power regardless of device orientation with respect to the vibration source. A multi- axis device can eliminate the need to manually align the harvester for maximum power. To achieve multi-axis energy harvesting, both out-of-plane and in-plane harvesters are realized on the same die. Out-of-Plane Energy Harvester The out-of-plane harvester comprises a seismic mass tethered to the substrate with integrated AlN-on-Si beam transducers. The mass captures low-frequency ambient vibrations in an out-of-plane translational mode (Fig. 2, top). Additionally, motion of the mass couples energy into the transducer high-frequency fundamental clamped- clamped beam mode (Fig. 2, bottom). To sense a high- strain region of the beam deflection, where the most charge is generated by the piezoelectric effect, the transducer electrodes and AlN cover the area near the substrate anchor point.
Figure 2: Out-of-plane harvester mode shapes (fmass = 149 Hz and fbeam = 13.7 kHz) and proposed method of operation.
1269
MEMS 2012, Paris, FRANCE, 29 January - 2 February 2012
In-Plane Energy Harvester The in-plane harvester consists of a seismic mass with free-standing cantilever micro-SLFNV ȝ-picks) located in the center of the mass (Fig. 3a). The mass exhibits translational motion in response to low-frequency vibrations (Fig. 3b). When external acceleration generates sufficient force, HDFK ȝ-pick can ³VQDS´ DQ $O1-on-Si spring transducer that subsequently vibrates at higher frequencies (Fig. 3c). The transducer top electrode connects regions of identical strain polarity to collect charge (Fig. 3d).
FABRICATION PROCESS FLOW Devices are fabricated on DȝP62,VXEVWUDWH (Figs. 4, 5, 6) using the AlN-on-Si process [7] with two modifications. First, the backside oxide mask is selectively etched to control seismic mass thickness, which enables any number of devices on the same wafer to be customized for a particular operating frequency. Secondly, HF release forms z-shock stops that ensure in- plane harvester ȝ-pick alignment.
Figure 4: Cross-section view of energy harvester on SOI substrate with Mo/AlN/Mo.
(a)
(b)
(c)
Figure 5: SEM images of out-of-plane harvester. AlN and top Mo are patterned to sense the fundamental beam mode, exposing bottom Mo in the beam center.
+ ++ ++++ _ _ _ _ _ _ _ _ _ _ _+ _+_+ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ +++ _ _ __ +++++ + + ++++ + + +_+__ _ _ _ _ _ _ _ _ _ _ _ _ __ +_ _+ + _ + _ + + _ _+ _+ _+ _+ _+ _+ +_ _+ +_ +_ +_ _+ + +_+_ _ _ _ _ +_ + + _ + + +++ + + + + ++++ _ _ _ _ +_ _ _ _ +_+_+ +
+ _ _ +
Relative strain Relative strain max (d) max Figure 3: Conceptual diagram of in-plane harvester: (a) rest position, (b) low-frequency seismic mass vibration, (c) high-frequency spring transducer vibration, (d) spring transducer strain patterns with charge formation for in- plane modes at 2.23 kHz (upper) and 6.24 kHz (lower). min
min
Figure 6: (upper left) SEM image of in-plane harvester ȝ- picks and spring transducer;; (lower left) z-shock stop removed to show handle Si;; (right) backside optical view.
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EXPERIMENTAL RESULTS Fabricated devices are mounted on a stage that provides sinusoidal out-of-plane or in-plane acceleration ain with frequency fin. The harvester output current is fed through a load resistance RL to a transimpedance amplifier (TIA). The TIA output voltage, which is proportional to the converted current, is measured by an oscilloscope. The out-of-plane harvester, with four beam transducer outputs connected in parallel, is first characterized over various accelerations at an input frequency close to the seismic mass resonance. A sinusoidal output signal fout = fin = 126 Hz is generated by the harvester, while increasing ain produces large transient current spikes occurring with frequency fspike in addition to the sinusoidal signal. When ain is increased to 0.7g, fspike also becomes equal to fin (Fig. 7).
Figure 7: Out-of-plane beam transducer output at various accelerations.
The output current impulses are found to have a main spectral component at fring 12 kHz (Fig. 8), which corresponds to the fundamental beam mode. A peak power of 3.23 nW is measured in response to a 134-Hz, 0.6g acceleration input.
Figure 8: Out-of-plane beam transducer output with periodic upconversion spikes;; (inset) close-up view of signal ring-down with 12kHz spectral component. A single spring transducer of the in-plane harvester is connected to the TIA, and similar acceleration characterization is performed at lower frequencies (Fig. 9). Based on preliminary measurements, a minimum acceleration of 0.45g is required to induce periodic actuations of the spring transducer with an fspike equal to fin. Missed actuations may result from imperfect ȝ-pick alignment due to slight tilting of the sample. For ain = 0.45g and fin = 2 Hz, complete actuations of one in-plane spring transducer occur with fspike = fin. A single upconversion spike produces a peak power of 2.27 nW (Fig. 10). Spectral analysis confirms signal components at fring = 2 kHz and 7 kHz contained in the upconversion spikes, which are in close agreement with simulated spring modes. While testing limitations prevented simultaneous measurement of both spring transducers, the total output current should nearly double as a result of the transducer symmetry [6]. The cantilever transducers on the seismic mass, which are also covered with piezoelectric material, can be measured in conjunction with the spring transducer output. Power calculations were based on measured current and RL. While RL = 560 : was used for in-plane acceleration characterization (Fig. 9), measurements with RL values up to 560 k: (Fig. 10) confirm that the output power can be maximized by selecting RL to approach the source impedance of the harvester.
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Figure 10: In-plane spring transducer output with upconversion spikes (RL = 560 k:);; (inset) close-up view of 7 kHz signal ring-down.
ACKNOWLEDGMENTS This project was sponsored in part by the Semiconductor Research Corporation through the Interconnect Focus Center, the National Nanotechnology Infrastructure Network (NNIN, USA), and the National Institute for Materials Science (NIMS, Japan). The authors also thank the Georgia Tech Nanoelectronic Research Center cleanroom staff for fabrication support.
REFERENCES
Figure 9: In-plane spring transducer output at various accelerations (RL = 560 :). CONCLUSION This work introduced micromachined multi-axis AlN-on-Si KEHs as a solution to scavenge low-frequency mechanical energy from the environment. Both out-of- plane and in-plane devices utilized integrated frequency upconversion transducers that increased the power output beyond the already-harvested energy at the tested input frequencies. The fabrication process accommodates for multiple degrees-of-freedom by incorporating out-of-plane and in- plane harvesters on the same substrate. Additionally, the fabrication process allows for multiple operating frequencies (and increased power output) by controlling the oxide mask thickness that determines the seismic mass thickness.
[1] D. E. Newland, "Pedestrian excitation of bridges²recent results," in Tenth Int. Congr. Sound Vib., 2003, pp. 1±15. [2] S. Roundy, "On the Effectiveness of Vibration-based Energy Harvesting," Journal of Intelligent Material Systems and Structures, vol. 16, p. 809, 2005. [3] R. Elfrink, et al., "First autonomous wireless sensor node powered by a vacuum-packaged piezoelectric MEMS energy harvester," in IEDM 2009, 2009, pp. 1-4. [4] C. B. Williams, et al., "Development of an electromagnetic micro-generator," Circuits, Devices and Systems, IEEE Proceedings, vol. 148, pp. 337-342, 2001. [5] T. Galchev, et al., "Non-resonant bi-stable frequency- increased power scavenger from low-frequency ambient vibration," in Transducers 2009, 2009, pp. 632-635. [6] I. Sari, et al., "An Electromagnetic Micro Power Generator for Low-Frequency Environmental Vibrations Based on the Frequency Upconversion Technique," J. Microelectromech. Sys., vol. 19, pp. 14-27, 2010. [7] W. Pan, et al., "Thin-film piezoelectric-on-substrate resonators with Q enhancement and TCF reduction," in MEMS 2010, 2010, pp. 727-730.
CONTACT *J. L. Fu, tel: +1-404-385-6693;; [email protected].
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